Add systemTimeZoneId() to the C++20 tzdb backend of QTimeZone

It's not strictly necessary, but it should be a little more
efficient. Without it, QTZ::systemTimeZoneId() falls back to using the
id() of a default-constructed backend, QChronoTZP(), which will get
the same answer returned here, but we can avoid the need for that
fall-back by doing the right lookup to begin with.

Change-Id: I6fe5beba048b5e9817b6bfed6eba785b13c2c206
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qtimezoneprivate_chrono.cpp

@@ -62,6 +62,15 @@ QChronoTimeZonePrivate::QChronoTimeZonePrivate(QByteArrayView id)
 QChronoTimeZonePrivate::~QChronoTimeZonePrivate()
         = default;
 
+QByteArray QChronoTimeZonePrivate::systemTimeZoneId() const
+{
+    if (const time_zone *zone = std::chrono::current_zone()) {
+        std::string_view name = zone->name();
+        return {name.begin(), qsizetype(name.size())};
+    }
+    return {};
+}
+
 QString QChronoTimeZonePrivate::abbreviation(qint64 atMSecsSinceEpoch) const
 {
     if (auto info = infoAtEpochMillis(m_timeZone, atMSecsSinceEpoch))

src/corelib/time/qtimezoneprivate_p.h

@@ -241,6 +241,8 @@ public:
     QChronoTimeZonePrivate(QByteArrayView id);
     ~QChronoTimeZonePrivate() override;
 
+    QByteArray systemTimeZoneId() const override;
+
     QString abbreviation(qint64 atMSecsSinceEpoch) const override;
     int offsetFromUtc(qint64 atMSecsSinceEpoch) const override;
     int standardTimeOffset(qint64 atMSecsSinceEpoch) const override;